This invention involves the separation of carbon dioxide and methane from gaseous mixtures wherein carbon dioxide is present in large quantities, i.e., from about 30 to about 90 mol percent.
2. Discussion of the Prior Art
The presence of carbon dioxide in admixture with hydrocarbons causes the heating value of the gas mixture to be diminished. To improve the heating value of the gas mixture, carbon dioxide must be removed. A considerable number of processes are available in the recovery of natural gas for the separation of carbon dioxide from such natural gases to make them of greater use. Most of these processes involve treatment of the gases with chemicals such as amines, propylene carbonate, potassium carbonate, N-methylpyrrolidone or other solvents. These processes are limited by the amount of carbon dioxide which can be efficiently and economically removed from the gas, some of the processes being applicable to gas streams having as much as 33 molecular percent of carbon dioxide. All of these processes require that the solution absorbing the gas be recovered through a stripping operation, thus materially increasing the cost of the process through not only the use of energy, but through the loss of chemical during the stripping step. Other known processes involve molecular sieve absorption or caustic treating, but such processes all suffer from the inability to economically separate the gases when large quantities of carbon dioxide are present. Cyrogenic processes have been used for gaseous separation and U.S. Pat. No. 3,595,782 describes such a process of separating carbon dioxide from ethane to leave a waste stream containing carbon dioxide and methane. Generally, cyrogenic processes have been thought to be inapplicable to carbon dioxide separation when large amounts are present because of the great tendency for carbon dioxide to form a solid at certain conditions, thus plugging the entire system and making any method inoperable.
While equilibria data of methane and carbon dioxide binary systems are readily available and well known, those skilled in the art have heretofore been unable to arrive at an economically feasible method which either did not require great amounts of energy consumption or avoid the freezing tendency of the carbon dioxide containing systems.
Surprisingly, a method has been discovered for separating carbon dioxide and methane from a mixture which contains from about 30 to 90 mol percent carbon dioxide without excessive energy consumption or freeze-up of the carbon dioxide in the plant equipment.